1. Field of the Invention
This invention relates to patterned media on hard disk drives and more particularly relates to guided growth of patterned media using monodisperse nanospheres.
2. Description of the Related Art
Hard-disk drives have rotating high precision aluminum or glass disks that are coated on both sides with a special thin film media designed to store information in the form of magnetic patterns. Electromagnetic read/write heads suspended or floating only fractions of micro inches above the disk are used to either record information onto the thin film media, or read information from it.
A read/write head may write information to the disk by creating an electromagnetic field to orient a cluster of magnetic grains in one direction or the other. Each grain will be a magnetic dipole pointing in a certain direction and also creating a magnetic field around the grain. All of the grains in a magnetic region typically point in the same direction so that the magnetic region as a whole has an associated magnetic field. The read/write head writes regions of + and − magnetic polarity, and the timing of the boundaries between regions of opposite polarity (referred to as “magnetic transitions”) is used to encode the data. To increase the capacity of disk drives, manufacturers are continually striving to reduce the size of the grains.
The ability of individual magnetic grains to be magnetized in one direction or the other, however, poses problems where grains are extremely small. The superparamagnetic effect results when the product of a grain's volume (V) and its anisotropy energy (Ku) fall below a certain value such that the magnetization of that grain may flip spontaneously due to thermal excitations. Where this occurs, data stored on the disk is corrupted. Thus, while it is desirable to make smaller grains to support higher density recording with less noise, grain miniaturization is inherently limited by the superparamagnetic effect.
In response to this problem, engineers have developed patterned media, where the magnetic thin film layer is created as an ordered array of highly uniform pillars, each pillar capable of storing an individual bit. Each bit may be one grain, or several exchange coupled grains, rather than a collection of random decoupled grains. In this manner, patterned media effectively reduces noise by imposing sharp magnetic transitions at well-defined pre-patterned positions, known as bit patterns. Bit patterns are organized as concentric data tracks around a disk.
One benefit of patterned media is the ability to overcome the above described superparamagnetic effect. Due to their physical separation and reduced magnetic coupling to one another, the magnetic pillars function as individual magnetic units, comprised either of single grains or a collection of strongly-coupled grains within each pillar. Since these magnetic pillars are typically larger than the individual grains in conventional media, their magnetization is thermally stable.
Conceptually, patterned media is a simple concept, however, mass producing disks at a reasonable cost is an immense challenge. To generate pillars or features required for the ever increasing data density, two approaches are taken: electron-beam lithography and nanoimprint replication. The main advantage of electron beam lithography, or e-beam lithography, over traditional photolithography is the ability of e-beam lithography to create nanometer-scale features. Traditional photolithography is limited because of the diffraction limit of light.
Unfortunately, e-beam lithography is a serial process, meaning that the e-beam must be scanned across the surface to be patterned. Therefore, e-beam lithography is not suitable for mass production. To overcome this limitation, a “master” disk is prepared by e-beam lithography and used to imprint “daughter” disks with nanometer scale features (hence (hence nanoimprinting).
E-beam lithography is capable of creating features having dimensions on the order of a few nanometers. However, the practical resolution of an e-beam generated feature is limited by forward scattering in the resist, backward scattering, and secondary electron travel in the resist. Each of the above can lead to a degradation of the resist and in some cases a complete removal of resist in the desired pattern area, and an uneven surface results.
Unfortunately, the uneven surface of the feature can affect the strength of the magnetic field of the feature. What is needed is a method for producing a smooth substrate utilizing e-beam lithography.